The invention relates to a device and method for the measurement of the thickness of a wet coating on a substrate. Existing methods and apparatus for determining the thickness of a coating of paint (pigmented, i.e. opaque liquid) or the like while it is still wet suffer from various disadvantages.
One known device for this purpose is the Pfund gauge. This consists of a spherically surfaced convex lens which is pressed onto the wet film until the centre touches the substrate. The diameter of the patch of paint thus formed on the surface of the lens may be used to estimate the thickness of the film. This type of gauge causes a large blemish on the surface of the film and can be rendered inaccurate by surface tension, or by the effect of particles in the paint or coating, also by curvature of the surface.
Another known gauge is the comb-type gauge. This has sections like the teeth of a comb where the ends of the teeth are cut back to different amounts. When the comb is pressed onto the surface, it is arranged that the end teeth are pressed down to the substrate and gaps of progressive size appear under the other teeth. Thus only those teeth are marked with paint which forms a gap of less than the thickness of the film of paint or coating. This type of gauge is difficult to check and is prone to wear of the teeth which touch the substrate. It gives great errors when used on surfaces which are not flat.